Multiple access communication network with dynamic access control

ABSTRACT

Dynamic access control for communicating a data packet over a multiple access communication network with a forward channel and a multiple access reverse channel which maintains utilization even when the arrival rate of packets for transmission exceeds the departure rate is provided by a broadcast dynamic access control parameter which regulates access to the multiple access reverse channel. A subscriber device wishing to transmit a data packet on the reverse channel receives the broadcast dynamic access control parameter from a base station on the network, generates an access control limit value and only attempts to transmit the data packet if the access control limit value satisfies the dynamic access control parameter. The dynamic access control parameter is updated by the base station based on a determined success rate and collision rate for transmissions on the reverse channel. The change rate for the dynamic access control parameter is based upon the determined success rate and a target success rate and the change direction is based on the determined collision rate and a target collision rate. Each reverse channel has an associated dynamic access control parameter and congested ones of the reverse channels are identified based on the value of the respective dynamic access control parameter and routing of data packets on the network is allocated to reduce demand on congested reverse channels.

FIELD OF THE INVENTION

This invention relates to communication networks and more particularlyto packet based communication networks with multiple accesscommunication channels having contention mode communications.

BACKGROUND OF THE INVENTION

Packet data systems are typically used for communications networks and avariety of protocols have developed to regulate transmission and receiptof data packets on such networks. As long as the ideal error-freecommunication channel has not been realized, data communication on suchnetworks generally allows, among other things, for retransmission ofdata when errors are detected. Various protocols governing datatransmission have evolved in an attempt to allow reliable datacommunications and to maximize network utilization. The need foroptimizing channel utilization becomes even more pronounced where aheavily utilized wireless communication network such as a cellular orPCS system or a wire line network comprises a portion of the datacommunications network.

Typical packet data systems include a forward channel, generallyreferred to as a downlink (from base to portable) in a wireless network,and a reverse channel, typically referred to as an uplink (from portableto base) in a wireless network. The forward (or downlink) channel istypically a constant stream of data messages which are broadcast from abase station to a population of listening subscriber devices. Theforward channel is a one to many broadcast channel. The reverse channel(or uplink) is shared by the population of subscriber devices and, as aconsequence, is often referred to as a many-to-one channel. Thisimmediately highlights that the access to the reverse channel by thesubscribers must be carefully managed due to the risk of multipledevices attempting to use the channel at the same instant, which resultsin collisions and a waste of the available bandwidth. The difficulty ofchannel management is exacerbated because a "listen before transmission"policy is generally hampered because the subscriber population isgeographically distributed. As a consequence, subscribers may notascertain if the reverse channel is currently being utilized by anothersubscriber. This is known as the hidden terminal problem.

Packet data multiple access protocols are generally classified into twocategories: contention protocols (such as Aloha and CSMA) or packetreservation protocols (such as PRMA). When employed in a digital packetradio system, these systems generally employ control flags that arebroadcast on the forward channel to inform the population of subscriberdevices the status of the reverse channel. This approach eliminates thehidden terminal problem since all subscribers receive the forwardchannel. Contention schemes usually utilize a small window in whichaccess is permitted. If a single transmission occurs, the control flagsinform the remaining members of the subscriber population not totransmit, allowing the channel to become dedicated to the transmittingsubscriber. However, if two transmissions occur at the same instant acollision occurs and channel bandwidth is wasted.

Packet reservation schemes manage the available channel bandwidth byeither polling each subscriber in turn and dedicating the reversechannel if the subscriber has messages to transmit, or by providing apredefined periodic short contention window portion of each block inwhich very short reservation requests may be transmitted. Upon receiptof a successful reservation request, the channel will be allocated tothe specific subscriber. The reservation transmission is regarded asvery efficient since wastage due to subscriber transmission collisionsgenerally does not occur.

In general, both categories of multiple access protocols havesignificant system advantages and disadvantages. The typicalconstruction of an Aloha Contention Protocol is illustrated in FIG. 1.As illustrated in FIG. 1, synchronization words are imbedded in theforward channel at regular intervals which, in turn, delineate thereverse channel into dedicated transmission slots. A subscriber deviceis permitted to transmit a data packet in any slot. As a consequence,the utilization of a slot may be classified as a success, idle, orcollision. A collision is defined when energy is present within slotboundaries but fails to decode which indicates that one or moretransmissions from different devices probably occurred. Idle slots aredevoid of energy from subscriber transmissions and successful receipt ofa transmission burst from a subscriber indicates successful utilizationof a slot. Collision and idle slots represent a waste of availablereverse channel bandwidth.

Aloha type contention schemes generally provide for inefficient reversechannel utilization where long data packet transmissions such as filetransfers are regularly encountered on the network. Such systems arealso prone to instability with increasing access rates resulting inincreased collisions followed by an increase in retransmission attemptswhich effectively continues to increase the demand on the network.Back-off policies are typically provided to reduce the risk of systeminstability. Aloha systems do, however, allow the use of slow receive totransmit switching time devices thereby allowing the use of low cost,half-duplex hardware.

A second type of contention scheme is a carrier sense multiple access(CSMA) protocol, the operations of which are illustrated in FIG. 2.Embedded in the forward channel is a sequence of control flags. Eachcontrol flag delineates the reverse channel into a sequence of collisionwindows and in addition indicates whether the reverse channel iscurrently being utilized (busy/idle) by a subscriber device. Asubscriber device which has an outstanding data packet(s) fortransmission determines if the channel is busy or idle prior totransmission. Furthermore, if the channel is sensed to be idle, thetransmission occurs such that the reverse channel synchronization wordis received by the base station prior to the transmission of the nextbusy/idle flag. This requirement ensures that the base station is ableto set the busy/idle flag to busy, thereby protecting the remainingsegments of the reverse channel subscriber transmission frominterference from other subscriber devices that may have outstandingdata packets ready for transmission. The technique allows the subscriberdevice to transmit variable length messages on the reverse channelwithout contending for the channel for each packet that requirestransmission. Collisions may, however, occur during the collision windowwhen the channel may be sensed to be idle by multiple subscribers withoutstanding packets.

A further variant on the CSMA protocol is available if full duplex isemployed. A subscriber device communicating on the reverse channel canthen monitor the forward channel during the transmission of a set ofdata packets. The forward channel control device such as the basestation can then alert a subscriber device if a collision has occurredand preempt termination of the transmission thereby conserving reversechannel bandwidth that may then be exploited by a different subscriberdevice. This approach is typically referred to as CSMA/CD (collisiondetect).

A disadvantage of the CSMA scheme is the need for fast receive totransmit switching times which typically means this protocol isinappropriate for use with inexpensive devices which may be desirable,particularly in a wireless environment. As CSMA is fundamentally acontention scheme, a back-off policy is typically provided as with Alohadue to the risk of system instability. An additional disadvantage ofCSMA schemes is that they typically provide relatively poor channelutilization for a very short transmission packet environments.

An example of a packet reservation type multiple access is illustratedin FIG. 3. The Packet Reservation Multiple Access (PRMS) schemes aretypically characterized by the partitioning of the reverse channel intoa polling region and a reservation/data region. The polling regionincludes short slots, with each slot dedicated to a specific subscriberor device. A device utilizes this slot to announce to the system that ithas outstanding data packets and that it requires a data slot to bereserved. If the forward channel device determines that a specific dataslot within the reservation region is not utilized then it may allocatethe particular data slot to a requesting reverse channel device fortransmission of the outstanding frames. Ordinarily, a single data slotis commensurate with the largest possible packet or transmission block.A reverse channel device may identify that the reverse channel has beenexclusively reserved for it by identifying that the reservationidentifier flag embedded in the forward channel has been set to itsidentification value. However, this approach wastes channel bandwidthfor very small messages, such as system acknowledgements and pollingslots that are left empty for each reverse channel device that is polledbut does respond because it does not have data packets to transmit. Suchschemes do, however, typically allow for the use of low cost slowerreceive to transmit switching speed technology.

Contention based protocols are subject to instability because collisionsmay occur which in turn require increased attempts due to retransmissionattempts. As traffic increases, utilization of the multiple accessreverse channel typically drops due to increased numbers of collisions.The collision messages then are submitted for retransmission whichcauses the actual attempt rate (as opposed to the new arrival rate) tosteadily increase without bound and once the optimal attempt rate hasbeen passed the system utilization steadily falls. Eventually, alltransmission slots may become filled with collisions and the systemutilization will reach zero. This results in both a loss of revenuebearing traffic for the channel and typically dramatically increases thedelay associated with delivery of data packets. This scenario isgenerally referred to as Aloha or reverse channel collapse.

To prevent Aloha instability, it is known to implement back-offprocedures in contention based protocols. A subscriber device is onlypermitted to re-transmit each packet a finite number of times.Furthermore, each re-transmission is required to be delayed by anexponentially increased delay. This back-off policy does not eliminatethe possibility that system instability can occur but the possibility issignificantly reduced. Furthermore, if the channel utilization does fallbecause the attempt rate has exceeded the maximum that the system cansupport, then the back-off policy provides a mechanism for recovery ifinstability occurs.

Back-off rules typically involve two components. First, if atransmission attempt fails then the subscriber device will delay asubsequent transmission attempt by a random time interval. Second, ifthe number of transmission attempts exceeds a predetermined thresholdthen the subscriber device will discard the queued packet and abort thetransmission attempt. The first rule minimizes the possibility that twoor more subscriber devices will execute re-transmission attempts afteran initial collision in an identical time slot. This approach providesan effective splitting algorithm that prevents continuous repeatingcollisions but it does not reduce the actual attempted traffic. Thesecond rule provides a form of non-persistence which allows the systemto recover. The rule effectively increases the departure rate, anddepartures are now partitioned between those that are successfullytransmitted and those that are abandoned.

The above stabilization procedure is generally only viable in systemswhere the contribution to the attempted traffic from new arrivals isessentially steady, predictable and sufficiently low so that the totalattempted traffic rate can remain at or below unity. The technique cancontrol short term transient increases in the arrival rate, which areassumed to be infrequent, and the associated loss in channel utilizationcan be tolerated. However, if the number of new arrivals exceeds thedeparture rate (both successful and aborted) then the system maycontinue to drift to lower utilization. However, even though such achannel is incapable of supporting the entire traffic volume,utilization may fall to where the channel is unable to provide optimalutilization for even a portion of the traffic volume because of thelimitations of these previously known stabilization techniques. Thisproblem is a particular concern for radio or wireless packet datasystems such as commercial, two-way, paging and message systems whichinclude a mix of short and extended message traffic. To summarize,reverse channel Aloha collapse is undesirable because the revenue streamgenerated by the cell is significantly reduced, subscriber devices burnexcess battery power through multiple fruitless transmission attemptsand message center originated messages will suffer an inordinateacknowledgment time while subscribers will be prevented from initiatingand successfully transmitting a data packet.

SUMMARY OF THE INVENTION

It is, therefore, an objective of the present invention to resolve theproblem of providing efficient utilization of and system stability for amultiple access channel in a packet data system including contentionbased access procedures under conditions in which the new arrival ratefor data packets to be transmitted exceeds the departure rate. Thepresent invention provides a multiple access communication network withdynamic access control allowing channel utilization to be optimized evenunder conditions in which the arrival rate of data packets fortransmission exceeds the system departure rate for such packages. Thepresent invention further provides a means for identifying congestedmultiple access channels so that traffic may be routed to less heavilyutilized channels.

A dynamic access control parameter is broadcast on a forward channel andreceived by all subscriber devices wishing to transmit on the associatedreverse channel. The dynamic access control parameter regulates accessto the reverse channel by defining a criteria which must be satisfied bya subscriber device before the subscriber device attempts to transmit adata packet on the reverse channel. The transmission parameter isgenerated based on the success rate and collision rate for the reversechannel. If the rate of successful data packet transmissions fails tomeet a desired level and the collision rate exceeds a desired level,then the dynamic access control parameter is adjusted to increase thenumber of transmission attempts by subscriber devices which areprevented. If the success rate fails to meet the predetermined criteriaand the collision rate is low, the dynamic access control parameter isadjusted to increase the percentage of transmission attempts bysubscriber devices which are allowed. By providing a dynamicallyadjusted control parameter which selectively limits the rate oftransmission attempts on the reverse channel, the utilization of thereverse channel (i.e., number of successful transmissions of datapackets on the reverse channel) can be optimized under networkconditions where the arrival rate of data packets would otherwise exceedthe departure rate causing a decrease in reverse channel utilization andsystem instability.

In one embodiment of methods of the present invention, a dynamic accesscontrol method for a multiple access communication network having aforward channel and a reverse channel is provided. Transmitted packetsare transmitted on the reverse channel to a forward channel transmissionapparatus or base station. The base station determines if the receivedpackets were received without error. A dynamic access control parameteris generated for regulating access to the reverse channel based onwhether the received packets were received without error. A channelcontrol packet (i.e. message) including the generated dynamic accesscontrol parameter is broadcast by the base station on the forwardchannel to control access to the reverse channel.

In another aspect of methods of the present invention, a transmissionsuccess rate and a transmission collision rate for the reverse channelof the communication network is determined. A change rate for thedynamic access control parameter is established if the determinedsuccess rate is below a predetermined acceptable level. A changedirection for the dynamic access control parameter is established basedon the collision rate. The dynamic access control parameter is thengenerated based on the established change rate and change direction. Asubscriber device receives the broadcast channel control packet andreads the dynamic access control parameter in the received channelcontrol packet. The subscriber device generates an access control limitvalue and submits a packet for transmission if the generated accesscontrol limit value satisfies the dynamic access control parameter. Theaccess control limit value in one embodiment of the present invention isbased on a random number function. The change rate is a function of thedetermined success rate and a target success rate and the changedirection is a function of the determined collision rate and the targetcollision rate.

In a further aspect of methods of the present invention, the dynamicaccess control methods include back-off procedures. A packettransmission attempt count is incremented after a packet is submittedfor transmission. The packet is transmitted if the attempt count is nogreater than a maximum attempt count. A transmission delay time is alsodetermined. The packet is transmitted after waiting the determinedtransmission delay time.

In another embodiment of methods of the present invention, a pluralityof base stations are connected to a network management system and thebase stations report their respective dynamic access control parametersto the network management system. Congested reverse channels areidentified based on the dynamic access control parameters and routing ofpackets on the communication network is allocated to reduce demand onreverse channels identified as congested.

Also provided are forward channel transmission apparatus or basestations for use in a multiple access communication network having aforward channel and a multiple access reverse channel. The forwardchannel transmission apparatus includes receiving means for receivingtransmitted packets on the reverse channel. The forward channeltransmission apparatus also includes determining means for determiningif the received packets were received without error. Generating meansgenerate a dynamic access control parameter for regulating access to thereverse channel based on whether the received packets were receivedwithout error. Broadcasting means is provided for broadcasting a channelcontrol packet including the generated dynamic access control parameteron the forward channel. The forward channel transmission apparatusgenerating means in one embodiment includes means for determining atransmission success rate and a transmission collision rate for thecommunication network and for establishing a change rate for the dynamicaccess control parameter if the success rate is below a predeterminedacceptable value. Means for establishing a change direction for thedynamic access control parameter based on the collision rate and forgenerating the dynamic access control parameter based on the establishedchange rate and change direction are also provided. In one aspect offorward channel transmission apparatus of the present invention, thechange rate is a function of the transmission success rate and a targetsuccess rate and the change direction is a function of the transmissioncollision rate and a target collision rate.

In another aspect of forward channel transmission apparatus of thepresent invention, the communication network includes a plurality ofmultiple access reverse channels each of which has an associated dynamicaccess control parameter and the forward channel transmission apparatusincludes means for comparing the dynamic access control parameters. Alsoprovided are means for identifying congested ones of the plurality ofreverse channels based on the dynamic access control parameters and forallocating routing of packets on the communication network to reducedemand on congested ones of the plurality of reverse channels.

Also provided are reverse channel transmission apparatus or subscriberdevices for use in a multiple access communication network having aforward channel including a broadcast channel control packet. Thebroadcast channel control packet includes a dynamic access controlparameter. The reverse channel transmission apparatus includes receivingmeans for receiving the broadcast channel control packet and readingmeans for reading the dynamic access control parameter in the receivedchannel control packet. Also provided in the reverse channeltransmission apparatus is generating means for generating an accesscontrol limit value and submitting means for submitting a packet fortransmission if the access control limit value satisfies the dynamicaccess control parameter.

In another aspect of reverse channel transmission apparatus of thepresent invention, the apparatus includes means for incrementing apacket transmission attempt count. Also provided in the reverse channeltransmission apparatus, are means for transmitting a packet if theattempt count is no greater than a maximum attempt count. Furthermore,means for determining a transmission delay time and means fortransmitting a packet after waiting the determined transmission delaytime are also provided. In another aspect of reverse channeltransmission apparatus of the present invention, the access controllimit value is based on a random number function.

The methods and apparatus of the present invention may be beneficiallyutilized in a variety of networks having a contention based multipleaccess channel. These networks may include wireless networks such ascellular networks or wire line networks. The methods and apparatus ofthe present invention may also be utilized in a wired network.

Accordingly, the dynamic access control methods and forward and reversechannel transmission apparatus of the present invention address theproblem of providing a reliable, high-utilization, multiple-accesscommunication channel whose utilization may be optimized even underconditions where the arrival rate of new data packets for transmissionexceeds the departure rate. The present invention further provides sucha system having the ability to reallocate traffic demand betweenchannels to reduce demands on overloaded channels. The methods andapparatus of the present invention are provided by utilizing a broadcastdynamic access control parameter which regulates access to a multipleaccess reverse channel by establishing a criteria which must be met bysubscriber devices before they are allowed to transmit data on thereverse channel. The dynamic access parameter is adjusted to optimizechannel utilization based upon a determined success rate and collisionrate for the multiple access channel of the communication network. Thedynamic access control parameter value from a number of differentforward channel transmission apparatus may be transmitted to a networkmanagement system which identifies congested reverse channels based onthe respective dynamic access control parameters and allocates routingof packets on the communication network to reduce demand on congestedchannels.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a slotted Aloha contention protocol.

FIG. 2 schematically illustrates a carrier sense multiple accessprotocol.

FIG. 3 schematically illustrates a packet reservation multiple accessprotocol.

FIG. 4 schematically illustrates an operating environment of the methodsand apparatus of the present invention.

FIG. 5 schematically illustrates an embodiment of combined contentionand reservation mode operations according to an embodiment of thepresent invention.

FIG. 6 is a schematic block diagram of a forward channel transmissionapparatus according to the present invention.

FIG. 7 is a schematic block diagram of a reverse channel transmissionapparatus according to the present invention.

FIG. 8 is a flowchart illustrating operations of a reverse channeltransmission apparatus according to a channel access method of thepresent invention.

FIG. 9 is a flowchart illustrating operations of a forward channeltransmission apparatus according to a channel access method of thepresent invention.

FIG. 10 schematically illustrates operations of an embodiment of thechannel access methods and apparatus of the present invention.

FIG. 11 schematically illustrates an embodiment of a channel accessmethod and apparatus of the present invention including a decode statusbit mask for acknowledgment of successfully transmitted packets and adynamic access control parameter.

FIG. 12 schematically illustrates an embodiment of a forward channelstructure according to the present invention.

FIG. 13 schematically illustrates an embodiment of a reverse channelstructure in contention mode according to the present invention.

FIG. 14 schematically illustrates an embodiment of the reverse channelstructure in reservation mode according to the present invention.

FIG. 15 schematically illustrates contention mode timing requirementsaccording to an embodiment of the present invention.

FIG. 16 schematically illustrates reservation mode access timingrequirements according to an embodiment of the present invention.

FIG. 17 schematically illustrates reservation/contention flag andreservation identification procedures according to an embodiment of thepresent invention.

FIG. 18 schematically illustrates an embodiment of dynamic accesscontrol according to the present invention.

FIG. 19 is a flowchart illustrating operations for dynamic accesscontrol according to an embodiment of the present invention.

FIG. 20 is a flowchart illustrating back-off operations for use with thedynamic access control methods and apparatus of the present invention.

FIG. 21 is a flowchart illustrating operations for determination of aprobability of transmission parameter for an embodiment of a dynamicaccess control method and apparatus according to the present invention.

FIG. 22 is a state diagram for a subscriber device according to anembodiment of the present invention.

FIG. 23 is a state diagram for a base station according to an embodimentof the present invention.

FIG. 24 schematically illustrates operations over time for an Alohasystem with back-off rules providing recovery.

FIG. 25 schematically illustrates operations over time for an Alohasystem where back-off rules fail to provide for recovery.

DETAILED DESCRIPTION OF ILLUSTRATED EMBODIMENTS

The present invention now will be described more fully hereinafter withreference to accompanying drawings, in which preferred embodiments ofthe invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

Referring now to FIG. 4, an operating environment of the presentinvention is schematically illustrated. Shown in FIG. 4 are fixed endsystems and a router device in a hardwired internet network 20. Hardwirenetwork 20 is connected to radio network controllers 22, 22'. Eachcontroller 22, 22' controls one or more base stations 24, 24'. Asillustrated in FIG. 4, base stations 24, 24' are radio communicationnetwork base stations associated with cells of cellular networksindicated as cell A, cell B, cell C, cell 1, cell 2 and cell 3,respectively. Base stations 24, 24' are forward channel transmissionapparatus which receive communications on one or more multiple accessreverse channels. Each base station 24, 24' may support one or morereverse channels.

Base stations 24, 24' establish radio data packet communications withsubscriber devices 26, 26'. Subscriber devices 26, 26' are reversechannel transmission apparatus which receive transmissions from basestation 24, 24' on a forward or downlink channel and transmit to basestations 24, 24' on a multiple access reverse or uplink channel.

Controllers 22, 22' and base stations 24, 24' may, for example, comprisea paging and messaging system. Controllers 22, 22' interface the radionetwork to hardwire network 20 and provide mobility management byconstructing and updating a directory or routing table to map aninternet IP address of subscriber device 26, 26' to a specific temporaryaddress and channel stream identifying a current connection forsubscriber device 26, 26' through one of base stations 24, 24'.Communications between internet fixed end systems and subscriber devices26, 26' may then be made transparent to the fixed end systems.

Overview of the Invention

Existing packet radio systems are designed to support specificapplications which exhibit well defined data packet sizes and trafficprofiles. Methods and apparatus of the present invention can support andadapt to a varying range of packet sizes and traffic profiles whileachieving a high channel utilization. This is desirable in apublic/commercial system which supports a multitude of differentapplications. Although it is generally difficult to predetermine theexact traffic profiles prior to commercial service, it is possible toidentify prominent characteristics. In general, data transactions areeither network 20 or subscriber device 26 originated. Network originatedtraffic primarily generates reverse channel traffic in the form of shortsystem acknowledgments indicating the receipt of a data packet. Incontrast, subscriber originated data transactions utilize the reversechannel to deliver messages of varying sizes. Exact packet sizestypically cannot be predetermined due to the unpredictable nature ofeach application that may utilize the system. In addition, as subscriberdevice 26 migrates throughout the radio network, very short locationupdate/cell transfer messages are preferably transmitted in an efficientmanner. Therefore, the reverse channel traffic is typically polarizedbetween a single, fixed size acknowledgment message and data packetmessages that are variable in size and have a somewhat larger minimumsize.

The present invention provides a contention mode suited to shortmessages and ensures that both upper layer management, connection andapplication protocols can correctly execute by transmitting longerframes. The inclusion of a contention free reservation mode enablesdevices to transmit extended messages that are not limited to a singledata packet. Furthermore, reservation mode allows the reverse channelutilization to arbitrarily approach 100% as the message length isextended.

The present invention consists of a forward channel data stream(broadcast by the base station) that is delineated into forward channelblocks (the data contained in each block is ordinarily protected againstchannel impairments and errors by an error detecting and correcting FECcode). The block boundaries are identified by the inclusion of a forwardchannel synchronization word within each block. Furthermore, embedded ineach block is an Reservation/contention flag and a ReservationIdentifier. The Reservation/contention flag is used to indicate to thepopulation of subscriber devices that the reverse channel is availablefor contention access or whether it has been allocated to a specificdevice for a reservation transmission.

The reverse channel transmissions may be categorized as a contentionburst, or as reservation block or multiple reservation blocktransmissions. The reverse channel transmissions are synchronized to theforward channel block intervals which may support a plurality ofcontention burst transmissions that are tightly synchronized to fallwithin slot boundaries, or a single reservation block transmission. Thecontention transmission bursts are designed to support acknowledgmentmessages to the forward channel traffic, reservation requests and allshort system protocol exchanges. The reservation block length isdesigned to support a single data packet (minimum length) that may betransmitted by a subscriber originated data transaction.

To enable slow receive to transmit (and vice versa) switching times, theforward channel Reservation/contention and Reservation Identifier flagscorrespond to reverse channel traffic that will occur in a subsequentblock(s). Preferably, a 2 block delay is provided to ensure that a lowcost radio technology may be employed. However, depending upon systemrequirements the delay may be reduced to a single block interval or,conversely extended to provide further switching time. Subscriber device26 secures a reservation transmission by first transmitting areservation request embedded in a contention burst to base station 24.The reservation request specifies the exact number of reverse channelblock intervals that are required to transmit an outstanding set of datapackets which constitute a subscriber originated transaction. If basestation 24 determines that the reservation may be granted, subscriberdevice 26 will observe that the forward channel Reservation/contentionflag and Reservation Identifier are set to indicate that the reservationtransmission may commence.

The present invention may be efficient because the reservation schemeallows an arbitrary unconstrained number of reverse channel blockintervals to be potentially assigned to a specific subscriber device 26.The invention need not sacrifice bandwidth by requiring that a fixedpredetermined number of block intervals cannot be fully utilized by thereservation request, as is common with existing reservation protocols.Moreover, the present invention may adapt to the specific trafficprofile observed by base station 26. That is, if the traffic profile isbiased towards contention traffic, the reverse channel is predominantlyoperated in the contention mode. Alternatively, if the reverse channeltraffic reflects a significant volume of extended length messages, thereservation mode is more heavily utilized.

Referring now to FIG. 5, the framing of the forward and reverse channelsaccording to an embodiment of the present invention is illustrated. Asillustrated in FIG. 5, the forward channel stream is delineated into aseries of transmission windows. Each transmission window includes asynchronization transmission, a Reservation/contention flag and aReservation ID. For purposes of understanding the present invention, thedescription shall be based on an Aloha type contention mode and theterms "contention" and "Aloha" will be used interchangeably throughoutthe balance of this detailed description unless specifically noted. TheReservation (resv.) or Aloha state for a corresponding subsequenttransmission window is indicated by bold. As illustrated in theembodiment of FIG. 5, the Reservation/Aloha flag in each blockcorresponds to a subsequent block, two blocks removed (i.e., block 1corresponds to block 3 transmission on the reverse channel). The reversechannel transmissions illustrated in FIG. 5 include short Aloha burstscorresponding to block 3 and reservation bursts corresponding to blocks7, 8 and 9. In the embodiment illustrated in FIG. 5, contentiontransmission windows are broken into 3 separate transmission slots.

Apparatus of the Present Invention

Referring now to FIG. 6, an embodiment of a forward channel transmissionapparatus or base station 24, according to the present invention, isillustrated. Base station 24 includes transmitter 30 or otherbroadcasting means for broadcasting a channel control packet on theforward channel. As illustrated in FIG. 5, base station 24 is acomponent of a radio data packet network including a forward channelhaving a wireless (radio) communication link in the forward and reversechannels. However, it is to be understood that the benefits of thepresent invention can also be obtained in a network including a wirelineor wired network so long as base station or forward channel transmissionapparatus 24 is operatively connected to the forward and reversechannels.

Receiver 32 or other receiving means for receiving a data packet and forreceiving a reservation request on the reverse channel are also includedin base station 24. Receiver 32 in the embodiment illustrated in FIG. 6is also operatively connected to a wireless (radio) reverse channel. Itis to be understood that while transmitter 30 and receiver 32 areillustrated as separate components capable of full duplex operation, thebenefits of the present invention may also be obtained using atransceiver combining the operations of transmitter 30 and receiver 32.Timer 34 or other synchronizing means provides for synchronizingoperations of transmitter 30 and receiver 32.

Base station 24 also includes packet receipt circuit 36 or other readingmeans responsive to receiver 32 for reading a reservation request todetermine the identifier associated with the requesting subscriberdevice 26. Packet receipt circuit 36 also reads data packets receivedvia receiver 32 from transmitting subscriber device 26. Channel controlcircuit 38 or other determining means responsive to packet receiptcircuit 36, determines if reservation mode access to the reverse channelis available to requesting subscriber device 26 and determines ifreservation mode or contention mode will be selected for the reversechannel in a subsequent transmission window. Packet transmit generationcircuit 40 or other generating means responsive to channel controlcircuit 38 generates the channel control packet including setting thereservation or contention mode indication and the identifier associatedwith a selected subscriber device 26. As illustrated in FIG. 6, packetreceipt circuit 36, channel control circuit 38 and packet transmitgeneration circuit 40 can be combined into base station control circuit42. Base station control circuit 42 interfaces to a data link entity(not shown) which receives and transmits data packets through basestation 24. Referring to FIG. 4, base station 24 may interface to afixed end system associated data link entity through controller 22.

In one embodiment of a forward channel transmission apparatus of thepresent invention, as illustrated in FIG. 6, channel control circuit 38further includes means for determining if a received data packet orreservation request has been successfully received. In this embodiment,packet transmit generation circuit 40 further includes generating meansfor generating a channel control packet which includes an acknowledgmentof successfully received data packets or reservation requests. Inanother aspect of the present invention, channel control circuit 38further includes means for queuing a plurality of received reservationrequests.

As illustrated in FIG. 5, the forward channel is delineated into forwardchannel blocks. In this embodiment, packet transmit generation circuit40, as illustrated in FIG. 6, includes means for broadcasting one of thechannel control packets in each of the forward channel blocks. As thereverse channel transmissions are synchronized to correspond to theforward channel blocks, timer 34 synchronizes receiver 32 andtransmitter 30 for reverse channel receipt timing. Furthermore, channelcontrol circuit 38 and packet transmit generation circuit 40 broadcastchannel control packets including an indication of reservation orcontention mode and an identifier which are associated with a subsequentone of the forward channel blocks. Similarly, indications in thebroadcast channel control packet of successfully received packets on thereverse channel are associated with a preceding one of the forwardchannel blocks.

As will be appreciated by those of skill in the art, the above describedaspects of the present invention in FIG. 6 may be provided by hardware,software, or a combination of the above. While the various components offorward channel transmission apparatus or base station 24 have beenillustrated in FIG. 6 as discrete elements, they may, in practice, beimplemented by a microcontroller including input and output ports andrunning software code, by custom or hybrid chips, by discrete componentsor by a combination of the above. For example, packet receipt circuit26, channel control circuit 38 and packet transmit generation circuit 40could all be implemented as a single programmable device.

Referring now to FIG. 7, an embodiment of a reverse channel transmissionapparatus or subscriber device 26 according to the present invention isillustrated. Subscriber device 26 includes receiver 44 or otherreceiving means for receiving a broadcast channel control packet.Receiver 44 also provides means for receiving data packets transmittedfrom base station 24. Subscriber device 26 further includes packetreceipt circuit 46 or other reading means for reading received channelcontrol packets to determine if a contention or a reservation mode isindicated and if an identifier associated with subscriber device 26 isincluded in the received channel control packet. In one embodiment ofthe reverse channel transmission apparatus of the present invention,packet receipt circuit 46 further includes means for reading anacknowledgment status flag in the channel control packet to determine ifa previously transmitted data packet was received.

Subscriber device 26 also includes channel control circuit 48 or otherdetermining means for determining if a data packet to be communicatedexceeds a predetermined size. Packet transmit generation circuit 50 orother means responsive to packet receipt circuit 46 and channel controlcircuit 48 for transmitting a data packet on the reverse channel eitherif contention mode is indicated and the data packet is no larger thanthe predetermined size or if reservation mode is indicated and anidentifier associated with the reverse channel transmission apparatus 26is included in the received channel control packet is also provided.Packet transmit generation circuit 50 further provides means responsiveto packet receipt circuit 46 and channel control circuit 48 fortransmitting a reservation request on the reverse channel if contentionmode is indicated and the size of the data packet to be transmittedexceeds the predetermined size.

Timer 52 or other synchronizing means is provided for synchronizing theoperations of receiver 44 and transmitter 54. Transmitter 54 operatesresponsive to packet transmit generation 50 to provide means fortransmitting data packets and reservation requests on the reversechannel. As illustrated in FIG. 7, packet receipt circuit 46, channelcontrol circuit 48, and packet transmit generation circuit 50 may becombined in subscriber device control circuit 56.

In one embodiment of the reverse channel transmission apparatus of thepresent invention as illustrated in FIG. 5, the forward channel isdelineated into forward channel blocks. Transmissions by transmitter 54are synchronized to correspond to the forward channel blocks as detectedfrom transmissions received by receiver 44 and packet receipt circuit46. Transmission of data packets and reservation requests by transmitter54 and packet transmit generation circuit 50 are provided in selectedsubsequent ones of the forward channel blocks which are associated withthe received channel control packets. Subscriber device 26 throughsubscriber device control circuit 56 is operatively connected to adatalink entity (not shown) which is receiving and transmitting datapackets over the radio communications network.

As will be appreciated by those of skill in the art, the above describedaspects of the present invention in FIG. 7 may be provided by hardware,software, or a combination of the above. While the various components ofreverse channel transmission apparatus or subscriber device 26 have beenillustrated in FIG. 7 as discrete elements, they may, in practice, beimplemented by a microcontroller including input and output ports andrunning software code, by custom or hybrid chips, by discretecomponents, or by a combination of the above. For example, packetreceipt circuit 46, channel control circuit 48, and packet transmitgeneration circuit 50 could all be implemented as a single programmabledevice.

Overview of the Methods of the Present Invention

Referring now to FIG. 8, an embodiment of operations for channel accessfor communicating a data packet over a multiple access communicationnetwork having a forward channel and a reverse channel according to thepresent invention will be described. More particularly, FIG. 8illustrates operations of a method according to the present invention asexecuted by a reverse channel transmission apparatus 26. At block 60,one of the broadcast channel control packets is received and read bysubscriber device 26 to determine if contention or reservation mode isindicated. If contention mode is indicated at block 62 and the datapacket size to be transmitted is no larger than the predetermined sizeat block 64, the data packet is transmitted as a short burst incontention mode at block 66. If it is determined that contention mode isindicated at block 62 and determined at block 64 that the data packet tobe communicated exceeds a predetermined size, at block 68 a reservationrequest is transmitted on the reverse channel. If reservation mode isindicated at block 62 and if the control packet read at block 60includes an identifier associated with the receiving subscriber device26 at block 70, the data packet or packets are transmitted inreservation mode at block 72.

In one embodiment of methods of the present invention, the controlpacket received at block 60 further includes an acknowledgmentindication (status flag) for previously transmitted data packets orreservation requests. The received acknowledgment status flagcorresponds to a preceding one of the channel transmission windows. Atblock 74, subscriber device 26 determines if it has any outstanding datapackets or reservation requests requiring acknowledgment. If so, atblock 76, subscriber device 26 determines from the acknowledgment statusflag or flags of the received channel control packet if the previoustransmission was received. At block 78, if the previous transmission isnot acknowledged, it is submitted for retransmission.

Referring now to FIG. 9, operations of an embodiment of the channelaccess methods of the present invention will be further described asexecuted by a forward channel transmission apparatus or base station 26.At block 80, a channel control packet is broadcast on the forwardchannel. At block 82, base station 24 monitors the reverse channel fortransmissions from subscriber devices 26. Base station 24 may receive atransmitted reservation request or a data packet on the reverse channelat block 82. A collision may also be detected at block 82. If it isdetermined at block 84 that a data packet rather than a reservationrequest has been received, the data packet is received and read at block86.

If it is determined at block 84 that a reservation request has beenreceived at block 82, the reservation request is read at block 88 todetermine the identifier associated with the subscriber device 26transmitting the reservation request. At block 90, base station 24determines if reservation mode access to the reverse channel isavailable to the subscriber device 26 transmitting the reservationrequest. At block 92, a channel control packet is generated fortransmission responsive to the received reverse channel transmission. Ifit is determined at block 90 that reservation mode access is availableto the requesting subscriber device 26, the generated channel controlpacket which will be subsequently transmitted at block 80, as indicatedby the looping path reflecting periodic transmission of channel controlpackets, is generated to indicate reservation mode and to include anaccess device identifier associated with the device transmitting thereservation request.

In one embodiment of methods of the present invention, the forwardchannel is delineated into forward channel blocks and the operations ofFIG. 9 are repeated for each forward channel block. In anotherembodiment of methods of the present invention, reservation requestsfrom subscriber device 26 further include an indication of the size ofthe data packet to be transmitted. Reservation mode access to any onerequesting subscriber device 26 is made available by base station 24 forno more than a predetermined maximum number of forward channel blocktransmission windows on the reverse channel for any single reservationrequest. This embodiment allows base station 24 to control the amount ofreservation mode access allocated to any individual subscriber device26. In another embodiment, reservation mode access operations at block90 include queuing a plurality of received transmitted reservationrequests for reservation mode access to the reverse channel.

In an additional aspect of methods of the present invention, asillustrated in FIG. 9, base station 24 provides acknowledgment ofreceived data packets. At block 86, when a data packet is successfullyreceived, an acknowledgment status flag is set which is included atblock 92 when the channel control packet for transmission is generated.The acknowledgment status flag in the generated channel control packetis associated with a specific previously transmitted data packet andindicates successful receipt of the data packet.

As is illustrated in the embodiment of FIG. 5, and more specificallywith respect to illustrated blk 3, the reverse channel blocks may bebroken into a plurality of contention mode transmission windows.Subscriber device 26 at block 66 and at block 68 of FIG. 8 selects oneof the predetermined number of contention mode transmission windowscorresponding to portions of the forward channel block designated forcontention mode transmission and transmits the data packet orreservation request, respectively, in the selected one of the pluralityof transmission windows.

Detailed Description of Operations of the Channel Access Methods andApparatus of the Present Invention

Referring now to FIG. 10, channel access procedures according to anembodiment of methods and apparatus of the present invention will now bedescribed. As illustrated in FIG. 10 at blk N-2, subscriber device 26examines the Reservation/contention (rsvr/Aloha) flag to determine itsstate, which is indicated in bold as Aloha in FIG. 10. Accordingly,subscriber device 26 transmits a short data packet or reservationrequest in a randomly selected one of the three Aloha burst slotsindicated as x, y, and z in blk N. Otherwise, subscriber device 26 waitsto transmit its request for a subsequent forward channel block. Bysending the reservation/contention flag two block intervals early,subscriber device 26 is allowed a minimum of a one full block intervalto switch from receive to transmit. It is preferable that subscriberdevice 26 selects one of the three Aloha burst transmission windows atrandom to minimize the risk of a collision with another transmittingsubscriber device 26.

If the data packet to be transmitted is too long for inclusion in asingle contention transmission window, a reservation request is sentindicating the amount of reverse channel bandwidth required to beallocated to subscriber device 26 to transmit the outstanding datapacket(s). If base station 24 accepts the reservation request anddetermines reservation mode access is available, subscriber device 26 isnotified via the forward channel two block intervals later as indicatedin FIG. 10 at blk N+2. Subscriber device 26 then determines that thereservation request has been successful by observing that thereservation/contention flag has been set to reservation and that thereservation identifier (id-subscriber in blk N+2) reflects theassociated identifier of the requesting subscriber device 26. This, onceagain, provides the minimum one block interval switching time fromtransmit to receive for subscriber device 26. The reservation flag isnot set to reservation by the forward channel transmission apparatus 24until blk N+2 (even though base station 24 may, as a full duplex devicehave received it, and been in a position to set the reservation flag inblk N+1) so that sufficient time is allowed for switching from transmitto receive by subscriber device 26. The data packet is then transmittedby subscriber device 26 beginning at blk N+4 and continuing with therequired number of additional blocks which, as illustrated in FIG. 10,include blk N+5 and blk N+6 for the reservation mode transmission fromsubscriber device 26.

As illustrated further in the embodiment shown in FIG. 11, methods andapparatus of the present invention further provide a low layer decodestatus mechanism to initiate correction of failed transmissions prior tothe invocation of higher layer recovery mechanisms to thereby reducechannel utilization for transmission of repeat request packets which aretypically required by higher layer recovery mechanisms. Lost datapackets may then be retransmitted and successfully received beforehigher layer recovery mechanisms initiate retransmission requests withthe associated overhead and channel utilization thereof.

As illustrated in FIG. 11, a channel access status octet is embeddedwithin each channel control packet transmitted in each forward channelblock. Typically, each of these transmissions is further protected by aforward channel error correcting strategy which strategies are known andunderstood by those of ordinary skill in the art and will not bediscussed further herein. As illustrated in FIG. 11, the channel accessstatus octet is partitioned with a six bit decode status bit mask and atwo bit channel access mode field. The first three bits of the decodestatus bit mask indicate the success or failure of the Alohatransmissions received on the reverse channel that correspond to theforward channel block interval transmitted two blocks prior to thecurrent forward channel block interval. The second three bits indicatethe success or failure of the Aloha transmissions received on thereverse channel that correspond to the forward channel block intervalthat was transmitted at three block intervals prior to the current blockinterval.

The second three bits of the decode status bit field represent statusinformation that is being retransmitted for the second time. Thisrepetition allows a subscriber device 26 that is, for example,experiencing a ten percent forward channel error rate to achieve a onepercent indeterminate rate in the decode status information.Accordingly, on receipt of a decode status failure or two indeterminatedecode status bit mask blocks, subscriber device 26 attempts to submitfor retransmission the data packet or reservation request. If an Alohaburst is not received or the base station services a reservation requestand receives reservation blocks, then the base station will set theDecode Status bits to failure.

A typical scenario is illustrated in FIG. 11. The following proceduremay be used to determine the setting of the Decode Status bits in theChannel Access Status word. For the purposes of this procedure, basestation 24 maintains a counter that increments as each forward channelblock is encoded and transmitted. Base station 24 sets the Decode Statusbits in the Channel Access Status word that is encoded and transmittedin the N^(th) forward channel Reed Solomon block according to thefollowing rules:

IF an Aloha burst was received in slot 1 on the reverse channel andcorrectly decoded while the (N-2)nd block was transmitted on the forwardchannel

THEN Base station 24 sets b₁₁ of the Channel Access Status word toindicate success (1)

IF an Aloha burst was received in slot 2 on the reverse channel andcorrectly decoded while the (N-2)nd block was transmitted on the forwardchannel

THEN Base station 24 sets b₁₀ of the Channel Access Status word tosuccess (1)

IF an Aloha burst was received in slot 3 on the reverse channel andcorrectly decoded while the (N-2)nd block was transmitted on the forwardchannel

THEN Base station 24 sets b₉ of the Channel Access Status word toindicate success (1)

IF an Aloha burst was received in slot 1 on the reverse channel andcorrectly decoded while the (N-3)rd block was transmitted on the forwardchannel

THEN Base station 24 sets b₈ of the Channel Access Status word toindicate success (1)

IF an Aloha burst was received in slot 2 on the reverse channel andcorrectly decoded while the (N-3)rd block was transmitted on the forwardchannel

THEN Base station 24 sets b₇ of the Channel Access Status word toindicate success (1)

IF an Aloha burst was received in slot 3 on the reverse channel andcorrectly decoded while the (N-3)rd block was transmitted on the forwardchannel

THEN Base station 24 set b₆ of the Channel Access Status word toindicate success (1)

IF a reservation block was received on the reverse channel while the(N-2)nd block was transmitted on the forward channel

THEN Base station 24 sets by b₁₁ b₁₀ and b₉ of the Channel Access Statusword to indicate failure (0)

IF a reservation block was received on the reverse channel while the(N-3)rd block was transmitted on the forward channel

THEN Base station 24 sets b₈ b₇ and b₆ of the Channel Access Status wordto indicate failure (0)

IF Base station 24 determines that specific reservation requestsreceived on the reverse channel while the (N-2)nd block was transmittedon the forward channel shall be discarded

THEN Base station 24 sets b₁₁ and/or b₁₀ and/or b₉ of the Channel AccessStatus word to indicate failure (0)

IF Base station 24 determines that specific reservation requestsreceived on the reverse channel while the (N-3)rd block was transmittedon the forward channel shall be discarded

THEN Base station 24 sets b₈ and/or b₇ and/or b₆ of the Channel AccessStatus word to indicate failure (0)

ELSE Base station 24 sets the decode status bits of the Channel AccessStatus word to indicate failure (0)

As illustrated in FIG. 11, the channel access mode orreservation/contention flag is provided as a two bit channel accessfield. This embodiment provides four operation modes to allow basestation 24 to adaptively set modes to reflect actual traffic flow on thenetwork. These modes, as illustrated in FIG. 11, include mode 1 (01), acontention mode with short reservation mode access only allowed (i.e.,for example, limited to one to two blocks of reservations transmissionper request). This mode may be desirable when the traffic profile for aparticular channel is heavily biased toward short numeric and alphapaging on the forward channel. A second mode, mode 2 (10) is contentionand reservation mode without queuing, but allowing longer reservationmode blocks. This mode is typically preferred when the traffic profilefor a particular cell is biased towards forward channel messaging.Allowing only one reservation request at a time to be accepted providestwelve contention burst occurrences between every reservationtransmission in the embodiment as illustrated in FIG. 9 where threeAloha bursts are allowed in each forward channel block. A final mode,mode 3 (11) is contention mode combined with queued reservations. Thismode is preferably utilized when the traffic profile for a particularcell is biased towards extended reverse channel messaging. It ispreferred that the actual traffic profile for a network be consideredfor each reverse channel stream and the specific operating mode selectedbe determined so as to optimize the reverse channel utilization.

Referring now to FIG. 12, the forward channel structure for anembodiment of methods and apparatus of the present invention will bedescribed. It is to be understood that the particular encoding and bitbreakdown of the illustrated forward channel structure is merelyillustrative.

As illustrated in FIG. 12, the forward channel includes a sequence of432 bit transmission blocks. Each block contains a 27 bitsynchronization sequence and a 27 bit reservation identifier which areinterleaved with a 378 Reed Solomon forward error correcting code. The 1bit Reservation/Aloha flag is "exclusive-or" ed with the 27 bit forwardchannel synchronization word.

The forward channel Reed Solomon block consists of parity symbols anddata symbols. The Reed Solomon code is a (63,51) code which ispartitioned between 12 RS parity symbols and 51 RS data symbols. Thedata field is partitioned between the data field that is utilized forsystem messages and the channel access status word, channel operationword and color code.

The channel access status word is a 12 bit/2 Reed Solomon symbol wordthat contains the decode status bits, channel mode bits and theprobability of transmission parameter. The channel operation word is a 4bit/2/3 Reed Solomon symbol word that contains the channel capacity flagand additional system specific parameters. The color code may beemployed in a radio environment to distinguish different transmissionchannels.

As illustrated in FIG. 13, the reverse channel Aloha transmission burstincludes a 132 bit transmission burst. Each Aloha burst contains a 6 bitsynchronization sequence that is split into two 3-bit words that areinterleaved with a 126 shorted Reed Solomon forward error correctingcode. The Reed Solomon code is a (63,57) code which is partitionedbetween 6 RS parity symbols, 15 RS data symbols and 42 all Zero RS datasymbols which are not transmitted. The Aloha transmission burst maycontain a Reservation request or a short data message. A non-uniformdistribution of the synchronization words throughout the Alohatransmission burst is provided. Additional synchronization words areplaced at the beginning of the burst to assist in rapid acquisition of atransmission from subscriber device 26 by base station 24.

As illustrated in FIG. 14, the reverse channel Reservation transmissionblock includes a 420 bit transmission block. Each Reservationtransmission block contains a 42 bit synchronization sequence that issplit into fourteen 3-bit words that are interleaved with a 378 ReedSolomon forward error correcting code. The Reed Solomon code is a(63,55) code which is partitioned between 8 RS parity symbols, 55 RSdata symbols. A non-uniform distribution of the synchronization wordsthroughout the Reservation transmission block is provided. Additionalsynchronization words are placed at the beginning of the burst to assistin rapid acquisition of a transmission from subscriber device 26 by basestation 24.

If subscriber device 26 has outstanding data packets for transmission,it is initially required to achieve synchronization with the forwardchannel and determine the reverse channel state from theReservation/Aloha flag and the channel access status word that isencoded within each forward channel FEC block. If the reverse channel isavailable for Aloha access, subscriber device 26 may proceed to attemptaccess to the reverse channel.

Subscriber device 26 will initially assess whether the outstanding datapackets may be compressed into a single Aloha transmission burst.Alternatively, subscriber device 26 will encode the data packets into asequence of Reservation blocks and construct an Aloha reservationrequest burst indicating the number of reverse channel block intervalsthat are required for the reservation transmission. The burst istransmitted in one of the three available Aloha slots. As illustrated inFIG. 15, to enable half duplex subscriber devices 26 that employ areceive to transmit (and vice versa) switch time that is commensuratewith a forward channel block duration, transmissions are delayed by twoforward channel block intervals. Although an extended time period isallocated for the device to switch from receive to transmit, the actualtransmission instant is preferrably relatively tightly specified. Thisensures that reverse channel bandwidth is efficiently utilized byallowing guard time, power ramp and synchronization overhead to beminimized.

Subscriber device 26 determines whether an Aloha transmission burst issuccessful by observing the decode status bits in the channel accessstatus word that is embedded in the forward channel Reed Solomon blocktransmitted two forward channel block durations after the Alohatransmission occurred. If the forward channel Reed Solomon block iscorrupted due to poor channel conditions, the decode status bits arealso replicated by base station 24 in the subsequent channel accessstatus word that is transmitted in the next forward channel Reed Solomonblock. If subscriber device 26 determines that the transmission wassuccessful, subscriber device 26 may terminate the channel accessprocedures. Alternatively, if the transmission attempt was a failure dueto the decode status bit indicating failure or was declaredindeterminate due to decoding failures, subscriber device 26 willinitiate a re-transmission attempt.

Depending upon the operating mode that is utilized by base station 24,subscriber device 26 may or may not be able to immediately determine thesuccess of the reservation request. When Mode 1 or 2 (see FIG. 11) areemployed, a single reservation request is serviced and additionalrequests for bandwidth are not queued. Therefore, the success of anAloha reservation request may be immediately determined by examining theReservation Identifier and Reservation /Aloha flag transmitted twoforward channel block intervals after the Aloha reservation request wastransmitted. However, in Mode 3 (see FIG. 11), reservations are queued.Thus subscriber device 26 is required to examine the decode status bitsand, if success is indicated, delay a re-transmission attempt of theAloha reservation request until it is confirmed that the reservationqueue is empty. If the reservation is granted, subscriber device 26 willinitiate transmission of the previously encoded Reed Solomon blocks. Ina similar manner to an Aloha transmission, subscriber device 26 isrequired to delay transmission by two forward channel block durations.

Subscriber device 26 timing requirements for the transmission of anAloha burst are also illustrated in FIG. 15. A significant period oftime is allocated to enable an extended receive to transmit switchingtime, receiver and DSP filter propagation delays, and demod/modulatorprocessing delays. However, despite the extended processing period, thetolerance of the transmission instant as illustrated in the embodimentof FIG. 15 is defined to an accuracy of ±0.25 bit times. This provides1.5 bit intervals (187.5 μs) to be allocated to free space propagationdelay.

Subscriber device 26 timing requirements for the transmission of a 428bit Reed Solomon reservation block are illustrated in FIG. 16. Asignificant period of time is allocated to enable an extended receive totransmit switching time, receiver and DSP filter propagation delays, anddemod/modulator processing delays. However, despite the extendedprocessing period, the tolerance of the transmission instant asillustrated in the embodiment of FIG. 16 is defined to an accuracy of±0.25 bit times. This provides 1.5 bit intervals (187.5 μs) to beallocated to free space propagation delay.

Referring now to FIG. 17, details of procedures for the use of theReservation/Aloha flag and Reservation Identifier of the channel controlpacket for an embodiment of the present invention will be furtherdescribed. The Reservation/Aloha flag and Reservation Identifier aretransmitted once in each forward channel block. The position of the 27bit Reservation Identifier and 27 bit Reservation/Aloha flag areillustrated in FIG. 17. The Reservation/Aloha flag as illustrated is"exclusive or" ed with the 27 bit forward channel synchronization word.

The purpose of the Reservation/Aloha flag is to inform the population ofsubscriber devices 26 that are currently registered or using the channelthat the reverse channel is available for slotted Aloha access on acontention basis, or that the reverse channel has been reserved for aspecific subscriber device 26. To provide subscriber devices 26 with atleast one forward channel block interval receive to transmit switchtime, the Reservation/Aloha flag is set two forward channel blocks inadvance of the actual associated reverse channel transmission window.

The Reservation Identifier is primarily utilized to identify a Specificsubscriber device 26 for which the reverse channel has been allocatedfrom a small subset of subscriber devices 26 for which reservationrequests may be queued. In a similar manner to the Reservation /Alohaflag, the Reservation Identifier is set two blocks in advance of theanticipated subscriber transmission. However, as illustrated in theembodiment of FIG. 17, the reservation Identifier is also set by basestation 24 to indicate that the reservation is in progress in the firstblock subsequent to receipt of a reservation request. When the system isoperating in Mode 2 (Aloha and Serve One at Random) this is utilized inconjunction with the Reservation/Aloha flag by the population ofsubscriber devices 26 to determine if the transmission of a reservationrequest should be temporarily delayed. This measure conserves subscriberdevice power consumption and channel capacity.

An additional feature of the procedures governing the ReservationIdentifier is also illustrated in FIG. 17. In the first and subsequentforward channel blocks after a subscriber specific ReservationIdentifier has been transmitted, base station 24 sets the ReservationIdentifier to the reservation in progress value. This prevents thesubscriber device 26 for which the channel has been reserved fromtransmitting exactly one or more blocks later than required if the firstReservation Identifier is missed. This is desirable because base station24 has reserved a specific number of reverse channel blocks for thatsubscriber device. Asynchronous transmission could potentially cause acollision with subsequent Aloha or queued reservation transmissions. Theprocedure also affords an additional advantage because the ReservationIdentifier, in combination with the Reservation/Aloha flag, may beemployed by the subscriber device 26 population to construct aneffective 48 bit Reservation/Aloha flag that is extremely robust andtolerant to an errored channel. This approach significantly minimizesthe number of occurrences in which a subscriber device 26 initiates anAloha transmission while a reservation session is currently in progress.

Base station 24 may operate in a variety of modes as illustrated in FIG.11. In general, the procedures for the Reservation/Aloha Flag areindependent of which operating mode is utilized by base station 24.Ordinarily, the Reservation/Aloha flag is set to reservation two forwardchannel block intervals prior to the reverse channel transmission bysubscriber device 26. Furthermore, the Reservation/Aloha flag is resettwo blocks prior to the termination of the reverse channel transmissionby subscriber device 26. The flag may be reset to indicate Aloha orReservation depending upon which operating mode is utilized by basestation 24. For the purposes of these procedures, base station 24maintains a counter that increments as each forward channel block isencoded and transmitted. A typical scenario is illustrated in FIG. 17.Base station 24 sets the Reservation/Aloha flag that is encoded andtransmitted in the Nth forward channel block according to the followingrules:

IF a reservation request was received on the reverse channel and grantedwhile the (N-2)nd block was transmitted on the forward channel

THEN Base station 24 sets the Reservation/Aloha Flag to indicateReservation

IF Base station 24 determines that a reserved transmission by asubscriber device 16 will terminate during the (N+1)^(st) forwardchannel block interval

THEN Base station 24 examines the reservation queue

IF Base station 24 determines that one or more reservation requests arestill outstanding

THEN Base station 24 resets the Reservation/Aloha flag to reserved

ELSE Base station 24 resets the Reservation/Aloha flag to Aloha

IF Base station 24 determines that a reserved reverse channeltransmission by a subscriber device 26 was absent during the N-1 forwardchannel block interval

THEN Base station 24 examines the reservation queue

IF Base station 24 determines that one or more reservation requests arestill outstanding

THEN Base station 24 resets the Reservation/Aloha flag to reserved

ELSE Base station 24 resets the Reservation/Aloha flag to Aloha

In general, the procedures for the Reservation Identifier are alsoindependent of which operating mode is utilized by base station 24.Ordinarily, the Reservation Identifier is set to identify a specificdevice two forward channel block intervals prior to the reverse channeltransmission by the identified specific subscriber device 26.Furthermore, the Reservation Identifier flag is reset two blocks priorto the termination of the reverse channel transmission by the identifiedsubscriber device 26. The identifier may be reset to indicate theidentification of the next subscriber device 26 for which the reversechannel has been allocated or reset to the null identifier dependingupon which operating mode is utilized by base station 24.

For the purposes of these procedures, base station 24 maintains acounter that increments as each forward channel block is encoded andtransmitted. A typical scenario is illustrated in FIG. 17. Base station24 sets the Reservation Identifier that is encoded and transmitted inthe N^(th) forward channel block according to the following rules:

IF A reservation request was received on the reverse channel and grantedwhile the (N-2)nd block was transmitted on the forward channel

THEN Base station 24 sets the Reservation Identifier to the reservationidentifier of the specific subscriber device 26 for which the reversechannel will be allocated.

IF Base station 24 determines that a reserved transmission by subscriberdevice 26 will terminate during the (N+1)^(st) forward channel blockinterval

THEN Base station 24 examines the reservation queue

IF Base station 24 determines that one or more reservation requests arestill outstanding

THEN Base station 24 resets the Reservation Identifier to thereservation identifier of the next subscriber for which the reversechannel will be allocated

ELSE Base station 24 resets the Reservation Identifier to the null value

IF A reservation request was received on the reverse channel and grantedwhile the (N-1)^(st) block was transmitted on the forward channel

THEN Base station 24 sets the Reservation Identifier to the reservationin progress value

ELSE IF The Reservation/Aloha Flag is set to reservation

THEN Base station 24 sets the Reservation Identifier to the reservationin progress value

ELSE Base station 24 sets the Reservation Identifier to the null value

Detailed Description of Dynamic Access Control Procedures of the PresentInvention

An embodiment of dynamic access control methods and apparatus accordingto the present invention will now be described. As described previously,dynamic access control procedures are beneficial to control channelbreakdown and recovery of multiple access channels which are subjectedto variable traffic loads to thereby optimize the channel utilizationunder varying load conditions. The basic Aloha access method allowssubscriber device 26 to unconditionally transmit an Aloha access burstin any slot. The dynamic access methods and apparatus of the presentinvention introduce a Probability of Transmission parameter, Ptx, thateliminates unconditional transmissions. For each transmission attempt,subscriber device 26 generates a random number between 0 and 1 (a fairdie is preferred)(note that the random number need not be a binary valuebut may be a selectively large range of values, albeit fractionalvalues, between 0 and 1). If this number is greater than the Ptxparameter that is broadcast in each forward channel block (see FIG. 11)by base station 24, transmission is permitted; otherwise subscriberdevice 26 does not transmit. Even though a transmission attempt was notmade, the subscriber back-off algorithm is executed as if anunsuccessful transmission attempt was actually made. This causessubscriber device 26 to exceed the maximum number of transmissionattempts threshold and discard the pending packet for a lower number ofactual unsuccessful transmission attempts.

In essence, as the attempted traffic rate rises, the dynamic accessmethods and apparatus of the present invention will adjust the Ptxparameter so that an increasing number of transmission attempts areactually diverted away from the channel so as to maintain the actualchannel attempt rate near unity so that optimal channel utilization canbe achieved. This allows the channel utilization to be maintained duringtransients when the attempt rate due to new arrivals and there-transmission of previously failed packets exceeds unity, or insituations when the arrival of new packets sustains a rate which is toohigh to be supported by the channel. This latter scenario is typical,for example, of the one-way paging busy hour just prior to noon in aradio based messaging and paging service environment. Using the methodsand apparatus of the present invention, even in cells which areover-subscribed the channel will not lose revenue due to loss of channelutilization. Furthermore, in one embodiment, base station 24 reports thePtx parameter to the Network Management system or controller 22 on aregular basis so the system provider may identify when cells arebecoming capacity limited and where new cells should be provided.

Prior to a transmission attempt, controller 24 preferably selects a cellthat enables the highest probability that a transmission will occur,that is the least busy cell, for the transmission. Furthermore,subscriber device 26 may elect to conserve battery power by not evenattempting a transmission where the Probability of Transmissionparameter indicates that a transmission attempt may be futile. Thetransmission attempt may be postponed to a later time when the channelis less busy.

The numerical precision of the Ptx parameter defines the accuracy towhich the channel utilization can be controlled. High numericalprecision theoretically would allow an Aloha channel utilization to beheld at a perfect 36% for any stable attempt rate that exceeds unity.However, because the attempt rate varies as a function of the trafficprofile, such precision is not typically warranted. Preferably, a 4-bitPtx parameter is utilized which is broadcast by base station 24 and thatdefines a range of uniformly distributed probabilities over the range 0to 1. Alternatively, for specialized traffic scenarios, the 4 bit indexmay be utilized to address a lookup table of 16 non-linear quantized Ptxvalues. Determination of the 16 values and whether they offer asufficient control range is dependent upon each application/protocol.However, preferably the selection of each entry should correspond to arange of attempt rates (G) that result in a channel utilization (S)which varies by only a few percentage points.

Utilizing a transmission probability parameter Ptx that is an unsigned 4bit binary integer (ranging 0-15), subscriber devices 26 are permittedto transmit if and only if a subscriber specific 4 bit unsigned randomnumber (generated for each transmission attempt) is less than or equalto the transmission probability, Ptx. Ordinarily, the Ptx parameter isset to 15 indicating that all subscriber Aloha transmission attempts mayproceed. However, if the population of subscriber devices 26 that sourceAloha attempts increases to a sufficient rate, such that the reversechannel utilization falls below an optimal value, then base station 24will adjust Ptx to ensure that only a supportable number of transmissionattempts are permitted to occur. It is to be understood that theparticular range of values used for Ptx is not critical but that thebenefits of the present invention may be obtained with any value rangein which subscriber device 26 selects one at random and only transmitsif the selected number satisfies the broadcast criteria value from basestation 24. In addition, specific subscriber devices 26 could be favoredor disfavored by providing them a non-random selection of the value forcomparison to the broadcast value.

Base station 24 preferrably sets Ptx so that a percentage of the Alohatraffic is diverted prior to an actual transmission attempt. This allowsbase station 24 to maintain the channel utilization during peak trafficperiods when the normal subscriber back-off rules are insufficient toprevent an Aloha collapse. This approach provides efficient utilizationof the reverse channel but does disadvantage (deny access to) a segmentof the subscriber device 26 population. The size of the disadvantagedpopulation increases as Ptx is reduced. A base station 24 that sets Ptxto a value less than 15 for a significant duration of time may beidentified as congested.

The Dynamic Access Control methods and apparatus of the presentinvention preferably provide that all subscriber devices 26 attemptingto access the reverse channel must utilize the latest, that is mostcurrent, value of the Probability of Transmission parameter, Ptx, thathas been estimated and broadcast on the forward channel by base station24. Referring now to the embodiment illustrated in FIG. 18, thebroadcast Ptx value is simply included in the data field of each forwardchannel Reed Solomon block. Alternatively, the broadcast PTx value maybe included in the channel access status word as illustrated in FIG. 11.

Referring now to FIG. 19, access procedures for subscriber device 26according to an embodiment of the dynamic access methods and apparatusof the present invention will now be described. If subscriber device 26has data packets to transmit then it is not permitted to make anunconditional transmission attempt as in a normal Aloha or existingcontention based multiple access protocol. Instead, as illustrated atBlock 100, subscriber device 26 obtains the latest value of theProbability of Transmission parameter, Ptx, that is broadcast on theforward channel. At Block 102, subscriber device 26 generates a randomnumber which is drawn from the same number space as Ptx. If the randomnumber exceeds the broadcast Ptx value at Block 104, the transmissionattempt is permitted at Block 106. Otherwise, the transmission attemptis treated as an unsuccessfull transmission attempt. If a packet istransmitted at Block 106 and determined not to have transmittedsuccessfully at Block 108 or if the random number does not exceed thePtx value at Block 104 an attempt counter is incremented at Block 110which is used in the back-off method as will be described. This causesan increasing number of transmission attempts to be aborted and packetsto be discarded as the Ptx parameter is reduced. Reducing Ptx enablesbase station 24 to regulate the number of actual reverse channeltransmission attempts.

An embodiment of back-off methods suitable for use with the presentinvention are illustrated further in FIG. 20. As illustrated in FIG. 20,if the attempt count is less than or equal to the maximum retry count, adelay is calculated at Block 114 as a function of the attempt count.Transmission is then delayed for the calculated time as illustrated atBlock 116. The packet is then submitted for transmission at Block 118.Preferably, prior to each re-transmission attempt (in the advent of atransmission failure), subscriber device 26 re-acquires the latest Ptxvalue that is broadcast on the forward channel and furthermore,generates a new random number to determine if the transmission attemptmay proceed. If the maximum count is exceeded at Block 112, the packetis discarded (treated as unable to be transmitted) at Block 120.

An embodiment of a method for calculating the Ptx value according to thepresent invention will now be described with reference to FIG. 21. Thefollowing terms will be used in the description:

Ptx Transmission Probability, determines the rate of access by apopulation of subscriber devices 26.

Soptimal Optimal Aloha success rate experienced by base station 24 onthe reverse channel (nominally 30%-40%). Soptimal is defined as the rateof successful Aloha bursts received during the Optimal Success RateUpdate Window.

Scurrent The current Aloha success rate observed by base station 24 onthe reverse channel (variable 0%-50%). Scurrent is defined as the rateof successful Aloha bursts received during the Ptx Adjustment Window.

Coptimal Estimated Aloha collision rate experienced by base station 24on the reverse channel (nominally 20%-30%) during the same period thatSoptimal is computed. Coptimal is defined as the rate of estimated Alohacollisions received during the Optimal Success Rate Update Window.

Ccurrent Estimated Aloha collision rate experienced by base station 24on the reverse channel (nominally 0%-100%) during the same period thatScurrent is computed. Ccurrent is defined as the rate of estimated Alohacollisions received during the Ptx Adjustment Window.

Reduction Factor Rate of decay for Soptimal, the parameter determinesthe longevity for which a set of reverse channel conditions and trafficprofiles are considered valid.

Exponent Determines the rate at which the Transmission Probability Ptxis updated.

Margin Defines a region of acceptable reverse channel utilization inwhich Ptx should not be adjusted.

Optimal Success

Rate Update Window Observation window defined by a specific number ofAloha slots, this window is used to estimate the channel parametersScurrent and Ccurrent.

Ptx Adjustment Window Observation window defined by a specific number ofAloha slots. Ptx Adjustment Window<Optimal Success Rate Update Window.

Avg RSSI Defined as the average received signal strength indicationassociated with successful Aloha bursts.

Step Internal algorithm parameter, sets the rate of change of Ptx.

At Block 130, base station 24 monitors the reverse channel to determinesuccessfully received packet and collision rates. The success andcollision rates for the reverse channel are calculated at Block 132.Success rate may be determined by counting each block Aloha slot thatsuccessfully decodes. The associated reverse channel collision rate isnot required to be an absolute estimate but is preferably a consistentestimate. The dynamic access methods and apparatus of the presentinvention are impervious to collision rate estimates that are constantlyunder or over estimated, but rely upon the underlying positive gradientmonotonic behavior of the collision rate as the attempt rate increases.When utilized in a wireless packet system, the embodiment illustrated inFIG. 21 declares a collision to have occurred when any detection (Aloha)slot exhibits an average RSSI that exceeds the average successful RSSIparameter, yet fails to be successfully decoded. Preferably, basestation 24 does not include decoding failures that may be attributed toco-channel interference or local oscillator leakage from subscriberdevices 26 located close to base station 24 in determining the collisionrate. When utilized in a wireline environment such as a wireless LAN, acollision is simply declared for each slot that fails to decode yetexhibits energy.

A predetermined optimal or acceptable success rate is defined for use atBlock 134. The acceptable success rate is preferably determined bycontinually assessing the reverse channel conditions and determining theoptimal success rate that can be supported by the reverse channel andthe associated collision rate experienced by the reverse channel. Inwireless systems, it is preferable that the system adapts to find theoptimal success rate value rather than specifying a fixed theoreticalvalue because the channel conditions may allow additional capacity to berealized through the capture effect or, alternatively, capacity may belost due to intense co-channel interference from a neighboring system.The reduction factor ensures that long term statistics are not retainedwhich may become obsolete as traffic and cell densities change. Aprefered technique for determining the acceptable or optimal successrate for use at Block 134 is as follows:

compute S_(current) and C_(current) by averaging the number of successesand collisions over a duration specified by the Optimal Success RateUpdate Window

if (Scurrent>Soptimal)

then Soptimal=Scurrent

Coptimal=Ccurrent

else Soptimal=max(Soptimal.Reduction Factor, 0.3)

The dynamic access methods and apparatus of the present inventionassesses the reverse channel conditions and determine the reversechannel utilization. If the utilization of the reverse channel is lowerthan that offered by the estimated maximum, then the methods andapparatus of the present invention determine if this is due to a highincidence of reverse channel collisions causing an Aloha collapse or ifit is simply due to a low attempt rate by the population of subscriberdevices 26. In the event of very high reverse channel attempt rates, theprobability of transmission parameter, Ptx, is reduced. Alternatively,if the attempt rate is low, Ptx is kept at a high value to encouragetransmission and hence greater utilization of the available reversechannel.

If the success rate is determined not to be acceptable at Block 134,then a change rate or step size for adjusting the value of Ptx isdetermined at Block 136. The step size may be determined as a functionof between the optimal and current success rates. For example, the stepsize may be set to the ratio of the optimal success rate to the currentsuccess rate raised to a selected EXPONENT value. However, it is to beunderstood that a variety of different methods for determining the stepsize may be beneficially used with the present invention.

If the collision rate is determined to be greater than a predeterminedoptimal or acceptable collision rate at Block 138, the value of Ptx isdecreased by the step size from Block 136 at Block 140. Otherwise, thevalue of Ptx is increased by the step size from Block 136 at Block 142.If the success rate is determined to be acceptable at Block 134, thevalue of Ptx is not changed. At Block 144, Ptx is broadcast by basestation 24 on the forward channel. If Ptx is broadcast as part of achannel control packet as described above, Block 144 corresponds to thebroadcast of the channel control packet as illustrated at Block 80 inFIG. 9 (See also FIG. 11 incorporating Ptx in channel access statusword). However, it is to be understood that the methods and apparatus ofdynamic access control according to the present invention may bebeneficially applied in a variety of contention based communicationsenvironments and are not limited to the inventive combined contentionand reservation mode channel access methods and apparatus describedherein. The operations of FIG. 21 may also be described for anembodiment of the present invention as follows:

compute ^(S) current and ^(C) current by averaging the number ofsuccesses and collisions over a duration specified by the Ptx AdjustmentWindow ##EQU1##

While not illustrated in FIG. 21, the MARGIN parameter is utilized toprevent the probability of transmission parameter, Ptx, from ditheringabout its true value. Packet attempt rates are typically quite variableand fluctuations in the order of a few percentage points are quitenormal. Consequently, the estimated collision and success rates willalso fluctuate. The MARGIN parameter ensures that the Ptx parameter doesnot fluctuate. The EXPONENT parameter is utilized to determine the rateof change of the Ptx parameter. These parameters ensure that Ptx can beadjusted rapidly to a significant and sudden increase in the reversechannel attempt rate such as that which may be encountered just prior tothe busy hour in a two-way messaging and paging system.

The dynamic access methods and apparatus of the present invention havebeen described herein as applied to a slotted Aloha system solely forthe purpose of fully describing the invention. Preferred values for theparameters described above for such an environment are provided in theTable 1 below:

                  TABLE 1                                                         ______________________________________                                        Parameter    Initial Value Value                                              ______________________________________                                        Soptimal      0.36                                                            Coptimal      0.26                                                            Reduction Factor           0.995                                              Exponent                   4                                                  Margin                     0.9                                                Optimal Success Rate       last 512 Aloha slots                               Update                                                                        Window                                                                        Ptx Adjustment Window      last 128 Aloha slots                               Avg RSSI     Determined from cell                                                          power product                                                    Ptx          15                                                               ______________________________________                                    

State Diagram Description of Channel Access Procedures

The basic state machine used by subscriber device 26 to control accessto the reverse channel is shown in FIG. 22. Preferably, this procedureshould not be violated when a congested forward channel is encountered.Utilization of the Transmission Probability, Ptx, parameter providessystem stability during periods in which excess transmission attemptsexceed the channel capacity.

If at any time the operation of this state machine is aborted, then allqueued data packets are preferably discarded and error messages issuedto the Data Link Layer entity indicating that the associated datapackets were undeliverable.

Furthermore, if the number of transmission attempts in the Aloha AccessState (2) exceeds the Max₋₋ Tx₋₋ Attempts threshold, then the channelstream is declared as congested and the transmission attempt isterminated. A transmission attempt consists of detecting and determiningthe value of the Reservation/Aloha flag while in the Aloha Access State(2).

IDLE STATE (1)

When the MAC layer of subscriber device 26 does not have data packets totransmit, it remains in the idle state (1). In this state the MAC layerentity does not attempt to access the reverse channel. Upon entry intothe idle state (1) the MAC layer entity remains in the state for aminimum of Min₋₋ Idle₋₋ Time forward channel Reed Solomon blockintervals. Min₋₋ Idle₋₋ Time is a system parameter that may beconfigured.

The MAC layer entity exits the idle state (1) and enters the alohaaccess state (2) on receipt of one or more data packets from the DataLink Entity. The MAC layer entity may, by implementation choice, decideto wait for a predetermined number of data packets to be queued or foran implementation specific time to elapse before a transmission isattempted. Furthermore, the subscriber MAC layer is only permitted toadd additional data packets to the transmission queue while in the idlestate (1).

Prior to exiting from the idle state (1), the subscriber MAC layer shallset a state variable of the No₋₋ Tx₋₋ Attempts to zero.

ALOHA ACCESS STATE(2)

Entry into the aloha access state (2) ordinarily occurs when the DataLink entity has outstanding data packets that require transmission.Alternatively, the aloha access state(2) can be entered due to an Alohaburst transmission failure or transmission attempt failure.

The subscriber MAC layer, upon receipt of data packets, assesses whetherthe data packets can be compressed to form a single Aloha transmissionburst. Alternatively, the MAC layer will encode the data packets into asequence of Reservation blocks and construct an Aloha reservationrequest burst indicating the exact number of reservation blocks forwhich reverse channel bandwidth should be allocated. Once the subscriberMAC layer has constructed an Aloha burst, it attempts to transmit theburst via the access procedures described herein.

If the state is re-entered due to an Aloha transmission failure ortransmission attempt failure then subscriber device 26 delaystransmission by implementing the back-off rules described previously.The back-off procedure is utilized to ensure that a repetition of thetransmission failure does not occur if the failure was due to a channelcollision between two or more subscriber devices attempting to utilizethe same Aloha slot. The procedure randomizes subsequent access to thereverse channel.

An embodiment of the procedures governing access by subscriber device 26will now be provided. For the purposes of these procedures subscriberdevice 26 maintains a counter that increments as each forward channelblock is received. The procedure is as follows:

IF the subscriber MAC layer has outstanding data packets fortransmission

THEN the subscriber will determine

IF the data packet(s) can be compressed into a single Aloha transmissionburst

THEN the subscriber device forms a single Aloha transmission burst

ELSE the subscriber device encodes the data packet(s) to form a sequenceof reservation blocks and forms an Aloha reservation request specifyingthe exact number of reservation blocks that are required to be allocatedby the base station

The subscriber MAC layer then selects at random a forward channel blockinterval in which the Aloha transmission burst will be transmitted. Theforward channel block interval is selected between Min₋₋ Count and 2No₋₋Tx₋₋ Attempts+1. The delayed block interval does not exceed Max₋₋ Count.This block interval is referenced as the N^(th) forward channel blockinterval. During the (N-2)nd forward channel block interval thesubscriber device shall examine the forward channel Reservation/Alohaflag, determine the transmission probability Ptx from the Channel AccessStatus word and generate a random number uniformly distributed between 0and 15.

IF the Reservation/Aloha flag indicates Aloha and the subscriber MAClayer has a queued Aloha transmission burst and the random number isless than the transmission probability Ptx

THEN the subscriber device transmits in one of the three available Alohaslots (selected at random) during the Nth forward channel blockinterval, increments the No₋₋ Tx₋₋ Attempts state variable and entersthe decode wait state (3)

ELSE IF the Reservation/Aloha flag indicates Aloha and the subscriberMAC layer has a queued Aloha reservation request and the random numberis less than the transmission probability Ptx

THEN the subscriber device transmits in one of the three available Alohaslots (selected at random) during the N^(th) forward channel blockinterval, increments the No₋₋ Tx₋₋ Attempts state variable and entersthe reservation wait state (4)

ELSE IF the No₋₋ Tx₋₋ Attempts exceeds Max₋₋ Tx₋₋ Attempts

THEN the subscriber device discards all outstanding data packet(s),issues an error message to the Data Link entity indicating that theoutstanding data packets were undeliverable and enters the idle state(1)

ELSE IF the entrance delay exceeds Max₋₋ Entrance Delay

THEN the subscriber device discards all outstanding data packet(s),issues an error message to the Data Link entity indicating that theoutstanding data packets were undeliverable and enters the idle state(1)

ELSE the subscriber device aborts the transmission attempt, incrementthe No₋₋ Tx₋₋ Attempts state variable and re-enters the aloha accessstate (2)

DECODE WAIT STATE (3)

When the subscriber MAC layer entity is in the decode wait state(3) itattempts to determine the success of the previous Aloha transmission.For the purposes of these procedures subscriber device 26 maintains acounter that increments as each forward channel block is received.Subscriber device 26 examines the Decode Status that is encoded andtransmitted in the N^(th) forward channel block and execute thefollowing procedures.

IF the subscriber device transmitted an Aloha burst in one of threeavailable Aloha slots in the (N-2)^(nd) forward channel block interval

THEN the subscriber device examines the corresponding decode status bitin the channel access status word embedded in the N^(th) forward channelblock

IF the decode status indicates that the subscriber Aloha transmissionwas successful

THEN the subscriber device enters the idle state (1)

ELSE IF the decode status indicates that the subscriber Alohatransmission was unsuccessful

THEN the subscriber device enters the aloha access state (2)

ELSE IF the forward channel block cannot be decoded due to an impairedblock

THEN the subscriber device examines the corresponding decode status bitin the channel access status word embedded in the (N+1)st forwardchannel block

IF the decode status indicates that the subscriber Aloha transmissionwas successful

THEN the subscriber device enters the idle state (1)

ELSE IF the decode status indicates that the subscriber Alohatransmission was unsuccessful

THEN the subscriber device enters the aloha access state (2)

ELSE the decode status is indeterminate due to two concurrent forwardchannel block errors

THEN the subscriber device issues an error message to the Data linklayer indicating that a potentially unsuccessful transmission of thedata packets has occurred and enters the idle state (1)

RESERVATION WAIT STATE (4)

When the subscriber MAC layer entity is in the reservation wait state(3)it attemps to determine the success of the previous Aloha reservationrequest transmission burst. For the purposes of these proceduressubscriber device 26 maintains a counter that increments as each forwardchannel block is received. Subscriber device 26 examines theReservation/Aloha Flag, Reservation Identifier and Decode Status that isencoded and transmitted in the N^(th) forward channel Reed Solomon blockand execute the following procedures.

IF the subscriber device transmitted an Aloha reservation request in oneof three available Aloha slots in the (N-2)nd forward channel blockinterval

THEN the subscriber device examines the Reservation/Aloha flag and theReservation Identifier in the N^(th) forward channel block

IF the subscriber device determines that the Reservation/Aloha flag isset to reservation and the Reservation Identifier matches the devicereservation identifier

THEN the subscriber device enters the transmit reservation access state(5)

ELSE IF the system is operating in Mode 1 or Mode 2.

THEN the subscriber device enters the aloha access state (2).

ELSE the system is operating in Mode 0 or Mode 3 and the subscriberdevice examines the corresponding decode status bit in the channelaccess status word embedded in the Nth forward channel block.

IF the forward channel block cannot be decoded due to an impaired block

THEN the subscriber device examines the decode status bits in thechannel access status word embedded in the (N+1)^(st) forward channelblock

IF the decode status indicates that the subscriber Aloha transmissionwas successful

THEN the subscriber device examines the Reservation/Aloha flag andReservation Identifier in each subsequent forward channel block.

IF the subscriber device determines that the Reservation/Aloha flag isset to reservation and the Reservation Identifier matches the devicereservation identifier

THEN the subscriber device enters the transmit reservation access state(5)

ELSE IF the subscriber device determines that the Reservation/Aloha flagis set to indicate Aloha access and the Reservation Identifier is set tothe null value

THEN the subscriber device enters the aloha access state (2)

ELSE the decode status indicates that the subscriber Aloha transmissionwas unsuccessful and the subscriber device enters the aloha access state(2).

TRANSMIT RESERVATION ACCESS STATE (5)

Upon initial entry into the transmit reservation access state (5), thesubscriber MAC layer entity waits for two forward channel blockdurations prior to transmission. During transmission, the subscriber MAClayer entity does not add additional data packets to the transmissionqueue. When all data packets have been transmitted, the subscriber MAClayer entity enters the idle state(1). For the purposes of theseprocedures, subscriber device 26 maintains a counter than increments aseach forward channel block is received. The procedures governing thisprocedure are detailed below.

IF the subscriber device determined that the Reservation Identifier inthe N^(th) forward channel block matched the subscriber reservationidentifier

THEN the subscriber device transmits a contiguous set of blocks; thefirst block will be transmitted during the (N-2)^(nd) forward channelblock interval. The number of transmitted blocks is identical to thenumber of blocks requested in the Aloha reservation requesttransmission. The transmitted reservation blocks contain the datapackets.

IF the subscriber device has completed transmission of the reservationblocks

THEN the subscriber device enters the idle state(1).

The basic state machine used by base station 24 for controlling accessto the reverse channel by the population of subscriber devices 26 isillustrated in the state diagram of FIG. 23. The setting of theReservation/Aloha flags and the Reservation Identifier occurs at thecorrect block boundaries.

CHANNEL IDLE STATE (1)

Initially, base station 24 sets the Reservation/Aloha flag to indicatethat Aloha transmission bursts may be received, sets the ReservationIdentifier to the null value, and enters the channel idle state (1).Base station 24 remains in the channel idle state (1) until a forwardchannel block boundary is reached:

IF the base station determines that one or more reservation requests arequeued for pending service and that the Reservation/Aloha flag has beenset to reservation for the previous two forward channel Reed Solomonblocks

THEN the base station enters the receive reservation blocks state (4)

else the base station enters the receive aloha bursts state (3)

RECEIVE ALOHA BURSTS STATE (2)

Base station 24 remains in the receive aloha bursts state (3) for theduration of the forward channel block period. During this time interval,base station 24 examines in turn each of the three reverse channel Alohaslots for the existence of an Aloha burst from a subscriber device 26.For each Aloha slot, base station 24 attempts to demodulate and detectan Aloha burst.

IF an Aloha burst is received error free or is successfully corrected

THEN the base station sets the corresponding decode status bits toindicate success

The base station 24 examines all received and correctly decoded Alohabursts for a MAC layer reservation request or a data packet. Reservationrequests and the associated subscriber reservation identifications willbe stored. Base station 24 remains in the receive aloha bursts state (3)until all three Aloha slots have been examined for the existence of asubscriber device 26 originated transmission:

IF the base station has examined all three Aloha slots and one or morereservation requests have been received

THEN the base station enters the reservation grant state (3)

ELSE the base station returns to the channel idle state (1)

RESERVATION GRANT STATE (3)

While in the reservation grant state (3) base station 24 examines thelist of reservation requests:

IF the base station is currently operating in Mode 0

THEN the base station executes system specific procedures

ELSE IF the base station is operating in Mode 1 (Aloha only)

THEN the base station ignores the reservation request and sets thecorresponding decode status bits to failure

ELSE IF the base station is operating in Mode 2 (Aloha and Serve OneReservation at Random)

THEN the base station examines the reservation queue

IF one reservation request is already pending service and the basestation has previously acknowledged the request

THEN the base station discards all reservation requests received in theprevious three Aloha slots and sets the corresponding decode status bitsfor each received reservation request to failure

ELSE the base station selects from the remaining reservation requestsone reservation identifier at random

Sets the forward channel Reservation Identifier to match the chosensubscriber and the Reservation/Aloha flag to reservation

Sets the corresponding decode status bits associated with the selectedreservation request to success

Sets the decode status bits for each received reservation request thatis not selected to failure

ELSE IF the base station is operating in Mode 3 (Aloha and QueuedReservations)

THEN the base station appends the reservation request(s) to thereservation queue

Sets the decode status bits associated with each reservation request tosuccess

Sorts the reservation queue so that reservation requests for shorterreverse channel transmissions are served in preference to longerreservation requests

Sets the forward channel Reservation Identifier to match the firstsubscriber in the reservation queue and the Reservation/Aloha flag toreservation

On completion of the above procedures the base station enters thechannel idle state (1)

RECEIVE RESERVATION BLOCKS STATE (4)

While in the receive reservation blocks state (4) base station 24attempts to detect and synchronize to the reverse channel subscriberdevice 26 transmission of reservation blocks. The transmission fromsubscriber device 26, if present, includes an embedded synchronizationsequence that is present in each of a predetermined number ofreservation blocks. The number of blocks will be identical to thatrequested in the initial reservation request and allocated by basestation 24.

IF the base station acquires synchronization with a reverse channeltransmission

THEN the base station attempts to decode a reservation block for eachallocated block that has been reserved

Procedures for Mode 2 (Aloha and Serve One Reservation at Random)Operation.

IF the base station is operating in Mode 2 and has successfully acquiredsynchronization with the subscriber transmission

THEN two blocks prior to the completion of the subscriber transmissionthe base station sets the Reservation/Aloha flag to Aloha and sets theReservation Identifier to the null value

On completion of the subscriber transmission the base station enters thechannel idle state (1).

Procedures for Modes 1 and 3 (Aloha and Queued Reservations) Operation.

IF the base station is operating in Mode 3 and has successfully acquiredsynchronization with the subscriber transmission

THEN two blocks prior to the completion of the subscriber transmissionthe base station re-examines the reservation queue

IF the reservation queue is empty

THEN the base station sets the Reservation/Aloha flag to Aloha and setsthe Reservation Identifier to the null value

On completion of the subscriber transmission the base station enters thechannel idle state (1).

ELSE the base station sets the Reservation Identifier to theidentification value of the next subscriber device in the reservationqueue and remains in the receive reservation blocks state (4)

IF the base station is operating in Mode 3 and determines in animplementation dependent manner that the subscriber transmission isabsent

THEN the base station maintains the Reservation/Aloha flag indicatingreservation, sets the Reservation Identifier to the identification valueof the next subscriber device in the reservation queue and remains inthe receive reservation blocks state (4)

System Stability

The benefits of the present invention may further be understood in thecontext of Aloha system performance. The dynamics of the stability issuemay be revealed if the throughput versus attempt rate response (S vs. Gcurve) of an existing slotted Aloha system is considered, as illustratedin FIG. 24. Misinterpretation of the system's performance occurs becauseit would appear that provided the attempt rate (G) is kept below unity,the channel utilization (S) would appear acceptable and stable.Unfortunately this static interpretation is flawed because it does notreflect the time based dynamic behavior of the system. The Aloha systemis stable if and only if the departure rate is equal to the arrivalrate. Consider the scenario when the attempt rate is unity. For every 10transmission attempts, approximately 3 are successful. The 7unsuccessful packets will become queued for re-transmission. During asubsequent time interval in which another 10 transmission attempts areexecuted, only 3 transmission attempts may correspond to new packetsthat have arrived if stability is to be achieved. If less than 3 newpackets arrive, the operating point on the Aloha response curve willdrift to the left and the channel utilization will fall. This is asatisfactory occurrence because although the channel is not beingutilized at its maximum, the delay experienced with the delivery of eachpacket is negligible and the system appears to provide excellent serviceto all users. However, if the number of new arrivals exceeds 3, thenumber of transmission attempts in a subsequent interval of time willexceed 10 and, as a consequence, an increase in the number of collisionswill be observed. This will cause the number of successful transmissionsto fall, which results in a decrease in channel utilization and afurther increase in the number of attempted re-transmissions during asubsequent time slot. Consequently, the operating point to drifts to theright of the optimum operating point. If the new arrival rate continuesto exceed the successful departure rate then the operating point willcontinue to drift to the right. This is a highly undesireable scenariobecause the channel utilization falls and, as a consequence, so does thelevel of revenue-bearing traffic. The delay associated with the deliveryof each packet dramatically increases and the quality of the system thatis perceived by the users falls. This scenario is often referred to asan Aloha or Reverse channel collapse.

To prevent Aloha instability, a subscriber device is only permitted tore-transmit each packet a finite number of times. Furthermore, eachre-transmission is required to be delayed by an exponentially increaseddelay. It is important to realize that this back off policy does noteliminate the possibility that system instability can occur; rather thepossibility is significantly reduced. Furthermore, if the channelutilization does fall because the attempt rate has exceeded the maximumthat the system can support, then the back off policy provides amechanism for recovery if instability occurs. The stability issue isnormally addressed by providing back off rules, such as:

IF a transmission attempt fails

THEN the subscriber device will delay a subsequent transmission attemptby a random time interval

IF the number of transmission attempts exceeds a predetermined threshold

THEN the subscriber device will discard the queued packet and abort thetransmission attempt

The first rule minimizes the possibility that two or more subscriberdevices will execute re-transmission attempts after an initial collisionin an identical time slot. This approach provides an effective splittingalgorithm that prevents continuous repeating collisions but it does notreduce the actual attempted traffic. The second rule provides a form ofnon-persistence which allows the system to recover and the operatingpoint to actually drift left towards the desirable operating region. Therule effectively increases the departure rate, and departures are nowpartitioned between those that are successfully transmitted and thosethat are abandoned. The non-persistence rule introduces a change in thesystem behavior. Referring now to FIG. 24, if the system is operating atthe optimum operating point and the successful departure rate is matchedby the attempted traffic of both new arrivals and re-transmissions, thenthe operating point will remain at this position. If a transientincrease in the attempted traffic rate occurs, then the operating pointwill drift to the right and the channel utilization will fall. If therate of new arrivals decays to the original rate, the non-persistencerule will slowly flush the excess packets queued for transmission andthe operating point will recover and drift left towards the optimumoperating point.

The above stabilization procedure is typically only viable in systemswhere the contribution to the attempted traffic from new arrivals isessentially steady, predictable and sufficiently low so that the totalattempted traffic rate can remain at or below unity. The technique cancontrol short term transient increases in the arrival rate, which areassumed to be infrequent, and the associated loss in channel utilizationcan be tolerated. FIG. 25 illustrates a scenario in which thisstabilization technique fails. If the number of new arrivals exceeds thedeparture rate (both successful and aborted) then the operating pointwill continue to drift to the right. This scenario could quite easilyoccur for a two-way paging and messaging system utilizing Aloha duringthe busy hour Oust prior to lunch) in an over subscribed downtown cell.Despite the fact that the channel is incapable of supporting the trafficvolume, it would be desirable for the channel to be utilized effectivelyby a subset of the subscriber population so that the service providercould still generate a revenue-bearing traffic stream. This capabilityis provided by the dynamic access methods and apparatus of the presentinvention.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

That which is claimed:
 1. A dynamic access control method for a multipleaccess communication network having a forward channel and a reversechannel comprising the steps of:receiving transmitted packets on thereverse channel; determining if the received packets were receivedwithout error; generating a dynamic access control parameter forregulating access to the reverse channel based on whether the receivedpackets were received without error; broadcasting a channel controlpacket including the dynamic access control parameter on the forwardchannel; and wherein said step of generating a dynamic access controlparameter includes the following steps:determining a transmissionsuccess rate and a transmission collision rate for the communicationnetwork; establishing a change rate for the dynamic access controlparameter if the determined success rate is below a predetermined value;establishing a change direction for the dynamic access control parameterbased on the collision rate; and generating the dynamic access controlparameter based on the established change rate and change direction. 2.The dynamic access control method of claim 1 wherein the following stepsare performed following said step of broadcasting a channel controlpacket:receiving the broadcast channel control packet; reading thedynamic access control parameter in the received channel control packet;generating an access control limit value; and submitting a packet fortransmission if the generated access control limit value satisfies thedynamic access control parameter.
 3. The dynamic access control methodof claim 2 wherein the following steps are performed following said stepof submitting a packet for transmission:incrementing a packettransmission attempt count; and transmitting the packet if the attemptcount is no greater than a maximum attempt count.
 4. The dynamic accesscontrol method of claim 2 wherein said step of generating an accesscontrol limit value includes the step of generating the access controllimit value based on a random number function.
 5. The dynamic accesscontrol method of claim 2 wherein the change rate is a function of thedetermined success rate and a target success rate and the changedirection is a function of the determined collision rate and a targetcollision rate.
 6. The dynamic access control method of claim 2 whereinthe communication network is a wireless network.
 7. The dynamic accesscontrol method of claim 6 wherein the wireless network is a cellularnetwork.
 8. The dynamic access control method of claim 2 wherein thecommunication network is a wireline network.
 9. A dynamic access controlmethod for a reverse channel transmission apparatus in a multiple accesscommunication network having a forward channel and a reverse channelcomprising the steps of:receiving a broadcast channel control packetincluding a dynamic access control parameter; reading the dynamic accesscontrol parameter in the received channel control packet; generating anaccess control limit value; submitting a packet for transmission if thegenerated access control limit value satisfies the dynamic accesscontrol parameter; incrementing a packet transmission attempt count;determining a transmission delay time; and, transmitting the packetafter waiting the determined transmission delay time if the attemptcount is no greater than a maximum attempt count.
 10. A forward channeltransmission apparatus for use in a multiple access communicationnetwork having a forward channel and a multiple access reverse channel,the apparatus comprising:means for receiving transmitted packets on saidreverse channel; means for determining if said received packets werereceived without error; means for generating a dynamic access controlparameter for regulating access to said reverse channel based on whethersaid received packets were received without error; means forbroadcasting a channel control packet including the dynamic accesscontrol parameter on the forward channel; and wherein said generatingmeans includes:means for determining a transmission success rate and atransmission collision rate for said communication network; means forestablishing a change rate for said dynamic access control parameter ifsaid success rate is below a predetermined value; means for establishinga change direction for said dynamic access control parameter based onsaid collision rate; and means for generating said dynamic accesscontrol parameter based on said change rate and said change direction.11. The forward channel transmission apparatus of claim 10 wherein saidchange rate is a function of said transmission success rate and a targetsuccess rate and said change direction is a function of saidtransmission collision rate and a target collision rate.
 12. A reversechannel transmission apparatus for use in a multiple accesscommunication network having a forward channel including a broadcastchannel control packet, said channel control packet including a dynamicaccess control parameter, said communication network further having amultiple access reverse channel, the apparatus comprising:receivingmeans for receiving said broadcast channel control packet; reading meansfor reading said dynamic access control parameter in said receivedchannel control packet; generating means for generating an accesscontrol limit value; submitting means for submitting a packet fortransmission if said access control limit value satisfies said dynamicaccess control parameter; means for incrementing a packet transmissionattempt count; means for determining a transmission delay time; and,means for transmitting a packet after waiting said transmission delaytime if said attempt count is no greater than a maximum attempt count.13. The reverse channel transmission apparatus of claim 12 wherein saidgenerating means includes means for generating said access control limitvalue based on a random number function.